Recursive Pattern Matching

Summary

Pattern matching extensions for C# enable many of the benefits of algebraic data types and pattern matching from functional languages, but in a way that smoothly integrates with the feel of the underlying language. Elements of this approach are inspired by related features in the programming languages F# and Scala.

Detailed design

Is Expression

The is operator is extended to test an expression against a pattern.

relational_expression
    : is_pattern_expression
    ;
is_pattern_expression
    : relational_expression 'is' pattern
    ;

This form of relational_expression is in addition to the existing forms in the C# specification. It is a compile-time error if the relational_expression to the left of the is token does not designate a value or does not have a type.

Every identifier of the pattern introduces a new local variable that is definitely assigned after the is operator is true (i.e. definitely assigned when true).

Note: There is technically an ambiguity between type in an is-expression and constant_pattern, either of which might be a valid parse of a qualified identifier. We try to bind it as a type for compatibility with previous versions of the language; only if that fails do we resolve it as we do an expression in other contexts, to the first thing found (which must be either a constant or a type). This ambiguity is only present on the right-hand-side of an is expression.

Patterns

Patterns are used in the is_pattern operator, in a switch_statement, and in a switch_expression to express the shape of data against which incoming data (which we call the input value) is to be compared. Patterns may be recursive so that parts of the data may be matched against sub-patterns.

pattern
    : declaration_pattern
    | constant_pattern
    | var_pattern
    | positional_pattern
    | property_pattern
    | discard_pattern
    ;
declaration_pattern
    : type simple_designation
    ;
constant_pattern
    : expression
    ;
var_pattern
    : 'var' designation
    ;
positional_pattern
    : type? '(' subpatterns? ')' property_subpattern? simple_designation?
    ;
subpatterns
    : subpattern
    | subpattern ',' subpatterns
    ;
subpattern
    : pattern
    | identifier ':' pattern
    ;
property_subpattern
    : '{' subpatterns? '}'
    ;
property_pattern
    : type? property_subpattern simple_designation?
    ;
simple_designation
    : single_variable_designation
    | discard_designation
    ;
discard_pattern
    : '_'
    ;

Declaration Pattern

declaration_pattern
    : type simple_designation
    ;

The declaration_pattern both tests that an expression is of a given type and casts it to that type if the test succeeds. This may introduce a local variable of the given type named by the given identifier, if the designation is a single_variable_designation. That local variable is definitely assigned when the result of the pattern-matching operation is true.

The runtime semantic of this expression is that it tests the runtime type of the left-hand relational_expression operand against the type in the pattern. If it is of that runtime type (or some subtype) and not null, the result of the is operator is true.

Certain combinations of static type of the left-hand-side and the given type are considered incompatible and result in compile-time error. A value of static type E is said to be pattern-compatible with a type T if there exists an identity conversion, an implicit reference conversion, a boxing conversion, an explicit reference conversion, or an unboxing conversion from E to T, or if one of those types is an open type. It is a compile-time error if an input of type E is not pattern-compatible with the type in a type pattern that it is matched with.

The type pattern is useful for performing run-time type tests of reference types, and replaces the idiom

var v = expr as Type;
if (v != null) { // code using v

With the slightly more concise

if (expr is Type v) { // code using v

It is an error if type is a nullable value type.

The type pattern can be used to test values of nullable types: a value of type Nullable<T> (or a boxed T) matches a type pattern T2 id if the value is non-null and the type of T2 is T, or some base type or interface of T. For example, in the code fragment

int? x = 3;
if (x is int v) { // code using v

The condition of the if statement is true at runtime and the variable v holds the value 3 of type int inside the block. After the block the variable v is in scope but not definitely assigned.

Constant Pattern

constant_pattern
    : constant_expression
    ;

A constant pattern tests the value of an expression against a constant value. The constant may be any constant expression, such as a literal, the name of a declared const variable, or an enumeration constant. When the input value is not an open type, the constant expression is implicitly converted to the type of the matched expression; if the type of the input value is not pattern-compatible with the type of the constant expression, the pattern-matching operation is an error.

The pattern c is considered matching the converted input value e if object.Equals(c, e) would return true.

We expect to see e is null as the most common way to test for null in newly written code, as it cannot invoke a user-defined operator==.

Var Pattern

var_pattern
    : 'var' designation
    ;
designation
    : simple_designation
    | tuple_designation
    ;
simple_designation
    : single_variable_designation
    | discard_designation
    ;
single_variable_designation
    : identifier
    ;
discard_designation
    : _
    ;
tuple_designation
    : '(' designations? ')'
    ;
designations
    : designation
    | designations ',' designation
    ;

If the designation is a simple_designation, an expression e matches the pattern. In other words, a match to a var pattern always succeeds with a simple_designation. If the simple_designation is a single_variable_designation, the value of e is bounds to a newly introduced local variable. The type of the local variable is the static type of e.

If the designation is a tuple_designation, then the pattern is equivalent to a positional_pattern of the form (var designation, ... ) where the designations are those found within the tuple_designation. For example, the pattern var (x, (y, z)) is equivalent to (var x, (var y, var z)).

It is an error if the name var binds to a type.

Discard Pattern

discard_pattern
    : '_'
    ;

An expression e matches the pattern _ always. In other words, every expression matches the discard pattern.

A discard pattern may not be used as the pattern of an is_pattern_expression.

Positional Pattern

A positional pattern checks that the input value is not null, invokes an appropriate Deconstruct method, and performs further pattern matching on the resulting values. It also supports a tuple-like pattern syntax (without the type being provided) when the type of the input value is the same as the type containing Deconstruct, or if the type of the input value is a tuple type, or if the type of the input value is object or ITuple and the runtime type of the expression implements ITuple.

positional_pattern
    : type? '(' subpatterns? ')' property_subpattern? simple_designation?
    ;
subpatterns
    : subpattern
    | subpattern ',' subpatterns
    ;
subpattern
    : pattern
    | identifier ':' pattern
    ;

If the type is omitted, we take it to be the static type of the input value.

Given a match of an input value to the pattern type ( subpattern_list ), a method is selected by searching in type for accessible declarations of Deconstruct and selecting one among them using the same rules as for the deconstruction declaration.

It is an error if a positional_pattern omits the type, has a single subpattern without an identifier, has no property_subpattern and has no simple_designation. This disambiguates between a constant_pattern that is parenthesized and a positional_pattern.

In order to extract the values to match against the patterns in the list,

  • If type was omitted and the input value's type is a tuple type, then the number of subpatterns is required to be the same as the cardinality of the tuple. Each tuple element is matched against the corresponding subpattern, and the match succeeds if all of these succeed. If any subpattern has an identifier, then that must name a tuple element at the corresponding position in the tuple type.
  • Otherwise, if a suitable Deconstruct exists as a member of type, it is a compile-time error if the type of the input value is not pattern-compatible with type. At runtime the input value is tested against type. If this fails then the positional pattern match fails. If it succeeds, the input value is converted to this type and Deconstruct is invoked with fresh compiler-generated variables to receive the out parameters. Each value that was received is matched against the corresponding subpattern, and the match succeeds if all of these succeed. If any subpattern has an identifier, then that must name a parameter at the corresponding position of Deconstruct.
  • Otherwise if type was omitted, and the input value is of type object or ITuple or some type that can be converted to ITuple by an implicit reference conversion, and no identifier appears among the subpatterns, then we match using ITuple.
  • Otherwise the pattern is a compile-time error.

The order in which subpatterns are matched at runtime is unspecified, and a failed match may not attempt to match all subpatterns.

Example

This example uses many of the features described in this specification

    var newState = (GetState(), action, hasKey) switch {
        (DoorState.Closed, Action.Open, _) => DoorState.Opened,
        (DoorState.Opened, Action.Close, _) => DoorState.Closed,
        (DoorState.Closed, Action.Lock, true) => DoorState.Locked,
        (DoorState.Locked, Action.Unlock, true) => DoorState.Closed,
        (var state, _, _) => state };

Property Pattern

A property pattern checks that the input value is not null and recursively matches values extracted by the use of accessible properties or fields.

property_pattern
    : type? property_subpattern simple_designation?
    ;
property_subpattern
    : '{' '}'
    | '{' subpatterns ','? '}'
    ;

It is an error if any subpattern of a property_pattern does not contain an identifier (it must be of the second form, which has an identifier). A trailing comma after the last subpattern is optional.

Note that a null-checking pattern falls out of a trivial property pattern. To check if the string s is non-null, you can write any of the following forms

if (s is object o) ... // o is of type object
if (s is string x) ... // x is of type string
if (s is {} x) ... // x is of type string
if (s is {}) ...

Given a match of an expression e to the pattern type { property_pattern_list }, it is a compile-time error if the expression e is not pattern-compatible with the type T designated by type. If the type is absent, we take it to be the static type of e. If the identifier is present, it declares a pattern variable of type type. Each of the identifiers appearing on the left-hand-side of its property_pattern_list must designate an accessible readable property or field of T. If the simple_designation of the property_pattern is present, it defines a pattern variable of type T.

At runtime, the expression is tested against T. If this fails then the property pattern match fails and the result is false. If it succeeds, then each property_subpattern field or property is read and its value matched against its corresponding pattern. The result of the whole match is false only if the result of any of these is false. The order in which subpatterns are matched is not specified, and a failed match may not match all subpatterns at runtime. If the match succeeds and the simple_designation of the property_pattern is a single_variable_designation, it defines a variable of type T that is assigned the matched value.

Note: The property pattern can be used to pattern-match with anonymous types.

Example
if (o is string { Length: 5 } s)

Switch Expression

A switch_expression is added to support switch-like semantics for an expression context.

The C# language syntax is augmented with the following syntactic productions:

multiplicative_expression
    : switch_expression
    | multiplicative_expression '*' switch_expression
    | multiplicative_expression '/' switch_expression
    | multiplicative_expression '%' switch_expression
    ;
switch_expression
    : range_expression 'switch' '{' '}'
    | range_expression 'switch' '{' switch_expression_arms ','? '}'
    ;
switch_expression_arms
    : switch_expression_arm
    | switch_expression_arms ',' switch_expression_arm
    ;
switch_expression_arm
    : pattern case_guard? '=>' expression
    ;
case_guard
    : 'when' null_coalescing_expression
    ;

The switch_expression is not permitted as an expression_statement.

We are looking at relaxing this in a future revision.

The type of the switch_expression is the best common type of the expressions appearing to the right of the => tokens of the switch_expression_arms if such a type exists and the expression in every arm of the switch expression can be implicitly converted to that type. In addition, we add a new switch expression conversion, which is a predefined implicit conversion from a switch expression to every type T for which there exists an implicit conversion from each arm's expression to T.

It is an error if some switch_expression_arm's pattern cannot affect the result because some previous pattern and guard will always match.

A switch expression is said to be exhaustive if some arm of the switch expression handles every value of its input. The compiler shall produce a warning if a switch expression is not exhaustive.

At runtime, the result of the switch_expression is the value of the expression of the first switch_expression_arm for which the expression on the left-hand-side of the switch_expression matches the switch_expression_arm's pattern, and for which the case_guard of the switch_expression_arm, if present, evaluates to true. If there is no such switch_expression_arm, the switch_expression throws an instance of the exception System.Runtime.CompilerServices.SwitchExpressionException.

Optional parens when switching on a tuple literal

In order to switch on a tuple literal using the switch_statement, you have to write what appear to be redundant parens

switch ((a, b))
{

To permit

switch (a, b)
{

the parentheses of the switch statement are optional when the expression being switched on is a tuple literal.

Order of evaluation in pattern-matching

Giving the compiler flexibility in reordering the operations executed during pattern-matching can permit flexibility that can be used to improve the efficiency of pattern-matching. The (unenforced) requirement would be that properties accessed in a pattern, and the Deconstruct methods, are required to be "pure" (side-effect free, idempotent, etc). That doesn't mean that we would add purity as a language concept, only that we would allow the compiler flexibility in reordering operations.

Resolution 2018-04-04 LDM: confirmed: the compiler is permitted to reorder calls to Deconstruct, property accesses, and invocations of methods in ITuple, and may assume that returned values are the same from multiple calls. The compiler should not invoke functions that cannot affect the result, and we will be very careful before making any changes to the compiler-generated order of evaluation in the future.

Some Possible Optimizations

The compilation of pattern matching can take advantage of common parts of patterns. For example, if the top-level type test of two successive patterns in a switch_statement is the same type, the generated code can skip the type test for the second pattern.

When some of the patterns are integers or strings, the compiler can generate the same kind of code it generates for a switch-statement in earlier versions of the language.

For more on these kinds of optimizations, see [Scott and Ramsey (2000)].